Nature's pharmacy

For centuries, native Americans have used the roots and leaves of Oenothera biennis - an edible plant with bright yellow, lemon-scented flowers - to treat wounds and respiratory disorders. Modern research, however, has focused on evening primrose oil (EPO) as a treatment for hormonal problems, schizophrenia and heart disease.

Made from the crushed seeds of the plant, EPO is often cited as the perfect herbal supplement. Rather than introduce harsh new chemicals - the "magic bullet" approach of orthodox medicine - it provides raw materials which the body then converts into the compounds it needs to stay healthy. What is more, EPO does not act alone. It works in combination with other vitamins and minerals, relying on the body's innate ability to use all these "building blocks" together to construct what it needs.

One group of compounds the body needs are the prostaglandins, hormone-like substances that play a part in everything from cell rejuvenation and the regulation of blood pressure, to how we feel when we get up in the morning. Normally, these chemicals are manufactured from linolenic acid - a fatty acid found in nuts, seeds and vegetable oils - but in some people the conversion process fails. Poor diet, stress, ageing, alcohol abuse and glucose intolerance can all compromise the production of prostaglandins, as can metabolic disorders such as diabetes. The solution is to take linolenic acid in the form of EPO, usually in glycerine capsules. This converts more reliably into prostaglandins.

EPO is also a source of gamma linolenic acid (GLA), responsible for the production of a particular prostaglandin, PGE1 - known for its anti-inflammatory and blood-thinning properties. GLA is also found in the oil of blackcurrant seed and borage.

In particular, EPO has been shown to be effective at treating premenstrual syndrome (PMS). In 1985 a trial reported in the Journal of Reproductive Medicine concluded that EPO was beneficial to women suffering from PMS, alleviating symptoms such as irritability and breast pain. This finding was backed by two American studies in 1987 but a 1994 paper in the British Medical Journal concluded that EPO was no more effective than a placebo at treating hot flushes.

In Canada, researchers have shown that EPO can lower blood pressure and protect the heart against chronic stress and atherosclerosis (narrowing of the arteries), while other studies have recommended it as a treatment for schizophrenia, and even for easing withdrawal symptoms in recovering alcoholics. Nevertheless, some critics say EPO's benefits have not been adequately researched. The PMS studies, in particular, are regarded as too subjective, relying on women's own impressions of whether their symptoms have improved.

Similar uncertainty surrounds the herb's side effects. Though considered safe in normal doses, excessive use of EPO can cause headaches and gastro-intestinal upset. It is also known to react badly with some orthodox drugs, particularly those used to treat epilepsy and schizophrenia. Indeed, three cases of seizure have been reported in schizophrenics taking evening primrose oil.

Because the herb has been shown to hinder platelet aggregation (stickiness of the blood), it may also increase the effect of blood-thinning medicines such as warfarin. Anyone taking anti-coagulants should talk to a GP before taking EPO. As with most herbs, it should not be taken when pregnant or while breast-feeding.

The herb, however, has a good safety record throughout centuries of documented folk use. Perhaps some reassurance can be had from the fact that the entire plant is edible and has never been known to cause ill effects when taken on its own. Externally, it has been used throughout history to treat wounds and abrasions. In his book Complete Herbal, the 17th-century apothecary Nicholas Culpeper declared the primrose family "as fine a salve to heal wounds as any that I know".

The following correction was printed in the Guardian's Corrections and Clarifications column, Thursday May 16 2002

Our references to "linolenic acid" should have read "linoleic acid" and "glycerine capsules" should have been "gelatine capsules". We neglected to mention two conditions for which evening primrose oil, the extracted seed oil (EPO) has been granted UK pharmaceutical licences, atopic eczema and mastalgia (breast pain).